By Larry Fine

NEW YORK (Reuters) - Roger Federer stormed into his fifth successive U.S. Open final on Saturday but his challenger will not be known for another 24 hours after heavy rain washed away the rest of the day's play at Flushing Meadows.

The four-times champion was in sublime form during a 6-3 5-7 7-5 6-2 victory over Novak Djokovic and can now enjoy an extra day of rest since world number one Rafael Nadal and British sixth seed Andy Murray were foiled by the bad weather.

Murray was in the driver's seat to book a date with the Swiss, leading Nadal 6-2 7-6 2-3, when play in the second semi-final was suspended by rain.

The 27-year-old Federer is now well-placed for his pursuit of a fifth Open title in a row and 13th grand slam crown since Murray and Nadal would need to complete their semi-final on Sunday.

Organisers gave up hope of finishing the second semi-final as the weather forecast called for hours of showers from the approaching Tropical Storm Hanna.

Because of the threatening weather, the matches were staged concurrently with four-times champion Federer facing Australian Open winner Djokovic at Arthur Ashe Stadium, and Nadal-Murray raging at adjacent Louis Armstrong.

The second semi-final, however, was started one hour 40 minutes after Federer and Djokovic's began, providing no chance of completing both as it turned out.

Federer produced his best performance of the tournament, hammering home 20 aces and ripping 51 winners in an emphatic two hour 45 minute victory over the third-seeded Serb.

The Swiss master, owner of 12 grand slam crowns yet thirsting for his first of 2008, was still on track to become the first man to win five successive Opens since Bill Tilden in 1924.

"Who do I prefer to play? I prefer the trophy, that's what I prefer," said Federer, deposed as world number one last month by Nadal after more than 4-1/2 years at the top spot.

"But I guess I would have to say Rafa because we've had such great battles over the year. Wimbledon was unbelievable so I hope we can have another one."

Federer's wish did not look likely to be granted as Murray belted his way to a two-sets advantage across the National Tennis Center plaza at the Armstrong stadium.

Nadal had not lost to the Scot in five previous meetings but the 21-year-old Briton was on song, displaying the form that had led him to three hard court titles this year.

Serving masterfully and dominating play from the baseline with his deeper groundstrokes, Murray swept through the first set in just over half an hour before clinching the second 7-5 in a tiebreak, racking up 44 winners along the way.

Nadal, however, broke Murray in the opening game of the third set to seize an early advantage as the French and Wimbledon winner desperately battled to reach his first Open final.

The Spaniard was darling of the crowd at the cosier Armstrong Stadium, which once held centre court for the Open before the towering Ashe Stadium opened in 1997.

At the start, the 10,000-seat stadium was only one-quarter full for the Nadal-Murray tilt while the 23,000-seat Ashe stadium was packed full of fans.

After Federer clinched his final berth, spectators crowded in while others unable to get a seat lined the lip at the top row of Arthur Ashe Stadium to peer down on the second semi-final.

(Editing by Pritha Sarkar)